Rat Respiratory Infections: Symptoms and Treatments

Definitions of Different Rat Respiratory Infections

There are a number of rat respiratory infections that you should be aware of when caring for your pet. These are common and oftentimes, chronic infections that can be lethal if not treated properly and immediately.

Bronchi (Bronchus): any of the major air passages of the lungs which diverge from the windpipe.

Bronchioles: any of the minute branches into which a bronchus divides.

Mycoplasma: a group of small typically parasitic bacteria that lack cell walls and sometimes cause diseases.

Mycoplasma pulmonis: a species-specific organism carried by nearly all pet rats, that colonizes the luminal surface and apical cell membranes of respiratory epithelium.

Murine Mycoplasmosis, also known as Murine Respiratory Mycoplasmosis (MRM), or Chronic Respiratory Disease (CRD). In most rodent species, MRM is a chronic and progressive disease, in which an infected rat can present with many variable clinical signs. Murine Mycoplasmosis is caused by mycoplasma pulmonis and is responsible for most respiratory and genital infections in rats.

Rhinitis: A viral infection or an allergic reaction can cause inflammation of mucous membranes in the nose.

Symptoms of Rat Respiratory Infections

Clinical signs of MRM will vary depending on the severity, site of infection, and age of the rat. Also, because the course of the disease is chronic, symptoms tend to increase gradually as the rat ages. Following symptoms that may be present include:

  • Symptoms of upper respiratory illness may range from no reaction to sneezing, snuffling, squinting, and porphyrin (rust-colored) staining around the eyes and nose. Another symptom is inner ear infections accompanied by head tilting, rolling, and face or ear rubbing.
  • In lower respiratory infections, the disease will advance along the respiratory passages causing bronchiolitis (inflammation of the bronchioles), bronchiectasis (abnormal widening of the bronchi or their branches), and bronchopneumonia (inflammation of the lungs, in the bronchi or bronchioles). The symptoms may include clattering moist breath sounds, labored breathing, gasping, wheezing, chattering, and coughing. Additional symptoms include hunched posturing, a rough coat, appetite, and weight loss, or behavioral change (e.g., nipping, biting, tiredness, avoidance).
  • Acute respiratory distress—increased respiratory rate, profound respiratory distress; may progress to respiratory arrest and death.
  • *Note: it is essential to mention that either upper and/or lower respiratory signs may be seen in CRD/MRM.
  • In genital infections, mycoplasma pulmonis can cause purulent endometritis (inflammation of the lining of the uterus), perioophoritis (inflammation of ovaries), and salpingitis (inflammation of fallopian tubes). The signs can range from clinically inapparent symptoms to abdominal distention or signs of blood-tinged uterine discharge. Hematuria (blood-tinged urine) from a concurrent urinary tract infection may also be present. Uterine infections attributed to Mycoplasma may also decrease litter sizes and health.


  • Chronic (long-term) airway inflammation initiated by bacteria and viruses.
  • Contact with other infected animals, although the probability of purchasing a mycoplasma-free rat, is extremely unlikely.
  • Poor husbandry, overcrowding, poor sanitation, and especially poor airflow will increase the risk for the development of the disease.
  • Other sources of immune system suppression (e.g., underlying liver or kidney disease) may predispose to clinical disease.
  • Obesity.

Etiology (manner or causation of disease)

The genus mycoplasma is a microscopic pleomorphic bacterium, from the family of Mycoplasmataceae. There are over 95 different mycoplasma species in nature. These also include several that can be found naturally in the mouth and genitourinary tract of humans and mammals.

Mycoplasma differs from other bacteria because it is enclosed by a cell membrane made of a lipid and protein bilayer. Atypical bacteria do not change color when gram-stained, but rather, remain colorless as they are neither Gram-positive nor Gram-negative. This is due to their lack of a cell wall. Because of this, antibiotics that inhibit cell wall assembly are ineffective against Mycoplasma and other atypical bacteria. Antibiotics that inhibit protein synthesis during prokaryotic mRNA translation are effective against Mycoplasma. This includes antibiotics such as aminoglycosides (gentamicin, amikacin), macrolides (azithromycin, erythromycin at bacteriocidal doses), and tetracyclines (doxycycline, minocycline), as well as those that disable bacterial enzymes such as fluoroquinolones (enrofloxacin).

Pathogenic Mycoplasma bonds best to the respiratory and urogenital tracts. Some Mycoplasma species have an inclination for articular cartilage. Mycoplasma species that cause disease in animals and humans can colonize on epithelial cell surfaces and attach to cell membrane receptors and superoxide radicals. This particular talent allows the bacteria to expose the host’s cells to peroxide metabolites. It is toxic to most cells due to its ability to non-specifically oxidize proteins, membranes, and DNA. This can lead to general cellular damage and dysfunction. Diseases caused by Mycoplasma tend to be chronic rather than suddenly occurring.

The mycoplasma bacteria carried by rats and mice are Mycoplasma pulmonis. The bacteria are host-specific and highly contagious among rats and mice. It’s transmitted through direct contact between mother and pups, intrauterine, sexually, and via short-distance aerosol transportation. M. pulmonis causes the most chronic disease in rats and mice. In fact, it can cause Murine Respiratory Mycoplasmosis and Murine Genital Mycoplasmosis.

Can Humans Catch M. Pulmonis?

No. M. pulmonis does not cause disease in humans. So, even though it can be carried by humans briefly in the nasal passages, it won’t infect them.

In addition, mycoplasmas are fragile organisms that rarely remain viable outside of a host for long periods. This makes it nearly impossible for them to be transmitted through cages, toys or carried on clothing. Transmission rates within a rat colony are based on husbandry practices, environment, and how many rats are in the colony.

Murine Mycoplasmosis (Murine respiratory mycoplasmosis, MRM) can be considered a syndrome. This means that although the etiology is Mycoplasma pulmonis, the disease is frequently initiated, accompanied, or exacerbated by other bacterial and/or viral infections.

Murine respiratory mycoplasmosis (MRM) is a gradually developing, chronic respiratory disease. Research indicates that while M. pulmonis will colonize the ciliated epithelium of the upper and lower respiratory tracts of young rats, the microscopic lesions cannot be identified until the rat approaches 2 to 6 months old. This means that chronic obstructive lung diseases may not display symptoms until the rat is 12-18 months old.

Because the infection is chronic, infected animals that are older, and/or immunocompromised, will often experience more severe signs of illness.

Factors that can contribute to the exacerbation of disease:

  • Stress and anxiety,
  • Secondary illnesses,
  • Over-crowding in cages,
  • Inadequate ventilation of the habitat, including ammonia buildup from urine and feces,
  • Obesity,
  • Bedding or litter that contains phenols (pine and cedar),
  • allergens such as incense, smoke, and vapor products.

Note: Two other murine mycoplasmas, M. arthritidis (causing arthritis) and M. neurolyticum (causing neurological issues in mice and rats) appear to have an antigenic heterogeneity (cross-reacts) with M. pulmonis. Mycoplasma pulmonis seems to be the only significant species of Mycoplasma that cause disease in rats, particularly repository diseases. Though Mycoplasmosis cannot currently be cured, responsible care and immediate treatment of symptoms will allow the animal to live a long and comfortable life.

Treatment of Rat Respiratory Infections

Before starting any treatment, consult your vet.

The following treatments list are of those that have been successfully used in rats to control signs of respiratory illness associated with mycoplasmosis. For information on individual drugs, dosages, and usage, see the Rat Medication Guide at the end of this lesson.

  • Set up a nebulization chamber at home.
  • Moderate exercise (not forced); can be useful in clearing secretions from the airways and will assist with weight loss. Limit if exertion causes difficulty breathing.
  • Weight loss is critical in obese rats, as it improves oxygen levels in the blood and decreases cough frequency.
  • Provide a balanced, nutritionally complete diet

For mild upper respiratory issues or rhinitis, you can use one of the following antibiotics to control symptoms.

Tylosin or the tetracyclines (tetracycline, doxycycline, minocycline) or enrofloxacin (Baytril). However, if signs become chronic or more severe with lung involvement, use an additional antimicrobial. Additionally, nebulization treatments or the use of corticosteroids may be required for an effective result.

For treatment regimens, see sections below: Early or mild illness, and Moderate to Advanced illness.

Early or Mild Rat Respiratory Infections

For early or mild infections suspected to be caused by Mycoplasma as the primary pathogen, we recommend:

  • Enrofloxacin (Baytril) dosage: 15 mg per kg of body weight, every 12 hours by mouth for 10 to 30 days or
  • Doxycycline 5 mg per kg of body weight every 12 hours by mouth for 10 to 30 days.
  • If they don’t respond within a reasonable timeframe (7-10 days), try both at the above doses for 10 – 30 days.

If a favorable response is not observed within a reasonable time frame, continue the medication as prescribed and add nebulization with the following mixture:

  • Nebulize 15-20 minutes, 2 to 3 times a day, for 14 days.
  • 8 mL sterile saline (amazon or local drug store)
  • 0.5mL Gentocin injectable 100 mg/mL (kvsupply.com or vet)
  • 0.5mL Albuterol 0.083% Inhalation (allivet.com or vet)
  • *Note: you can refrigerate any excess nebulizer mixture for up to 3 days.

If you suspect mild lung involvement is present, or the animal is not responding to antibiotics or nebulization, add dexamethasone (chewy.com) at the following dosage schedule:

Add dexamethasone to the treatment regimen. At 0.5 mg/lb, wean down the dosage as follows:

  • 0.5 mg per lb body weight twice daily injectable or by mouth for 3 days.
  • 0.5 mg per pound of bodyweight once a day for 3 days.
  • 0.25 mg per pound of bodyweight once a day for 3 days.
  • 0.25 mg per pound of bodyweight, orally every other day, three doses.

Moderate to Advanced Rat Respiratory Infections

If there is a poor or no response to one of the above medications, or if illness or symptoms advance to more serious lower respiratory infections or include middle to inner ear infections, more aggressive medications such as azithromycin (Zithromax) (revivalanimal.com), chloramphenicol (chewy.com), or combination therapies such as enrofloxacin (Baytril) plus doxycycline, or azithromycin plus doxycycline, or the addition of gentamicin (an aminoglycoside) can be administered.

You should note that enrofloxacin (a bactericidal antibiotic) plus doxycycline ( a bacteriostatic antibiotic) are listed as one of the combinations to be used in mycoplasmosis infection. It is true that Normally, bactericidal and bacteriostatic agents are not commonly used in conjunction. However, their efficacy in controlling illness in rats, due to mycoplasmosis has been documented and thus recommended.

The following treatment regimen is for moderate, advanced, or severely difficult-to-treat Mycoplasma-inflicted respiratory illnesses:

  • Enrofloxacin (Baytril) 15 mg per kg of body weight, every 12hr by mouth for 10-30 days.
  • Doxycycline 10 mg per kg of body weight every 12hr by mouth for 10-30 days.
  • Use Nebulizer for 15-20 minutes, 2 to 3 times a day, for 14 days using the nebulizer directions located above.
  • Dexamethasone 1 mg per lb body weight, twice daily, then wean down as follows:
  • 0.5 mg per lb body weight twice daily injectable or by mouth for 3 days.
  • 0.5 mg per pound of bodyweight once a day for 3 days.
  • 0.25 mg per pound of bodyweight once a day for 3 days.
  • 0.25 mg per pound of bodyweight, orally every other day, three doses.
  • If your vet’s recommended dexamethasone dosing and reduction schedule differ from ours, defer to your vet’s recommendations. This applies to all medical advice.

Bronchodilators and Corticosteroids

The use of bronchodilators such as Aminophylline and Theophylline ether orally or injectable, or Albuterol (nebulized in normal saline or with 7% hypertonic saline which can help break down mucus) may be added to help relax the muscles and dilate the bronchi in the lungs to aid breathing.

Nebulized treatments involving antibiotics should always include your vet.

If your rat has difficulty taking medicine orally, use antibiotics such as enrofloxacin (Baytril), gentamicin (Gentocin), or Tylosin. Remember, dilute nebulization medicines in normal saline unless otherwise directed by your veterinarian.

The following are the recommended ratios for dilution of medications in a normal saline solution:

Enrofloxacin (Baytril) 10mg to 1 mL normal saline.

Gentamicin (Gentocin) 5mg to 1 mL normal saline.

All others a 1 to 10 solution.

For information on nebulizers or how to nebulize, see the Nursing Care Section.

Add corticosteroids (prednisone or dexamethasone) to a treatment regimen to help reduce inflammation of the bronchi and bronchioles and help the rat breathe easier.

Please Note

Dexamethasone has been shown in controlled studies to reduce plasma leakage, resolve Mycoplasma-induced inflammation, and reduce the number of Mycoplasma organisms when used as the sole therapy.

It should also be noted that based on dexamethasone’s long biologic activity, it is advised to use the lowest dose that will achieve the desired effect.

Treatment Length and Maintenance

As Mycoplasma can probably never be completely eliminated from the airways, so it may be necessary to extend antibiotic treatment for 6 to 8 weeks. The antibiotics used to control the symptoms will allow the animal’s damaged airways time to heal. Consider continuing a maintenance schedule at a reduced dose. To control Chronic Mycoplasmosis, employ pulse antibiotic therapy or long-term intermittent dosing. For appropriate dosing when using pulse antibiotic therapy for chronic illness, discuss with your veterinarian.


It is essential always to complete a medication treatment or regimen prescribed. However, if your rat shows no improvement within five days of starting treatment, or should the condition worsen at any point during treatment, see your veterinarian to discuss any alternative antimicrobial or appropriate combination therapies.

Always give the correct dose of the medication for the full prescribed length of time to prevent bacterial resistance developing. Don’t skip doses.

Treatment of Rat Respiratory Infections in Pregnant or Nursing Females, and Rats Less than 4 Months Old

Recommended antimicrobials are azithromycin or Tylosin.

Note: The use of enrofloxacin and doxycycline is not recommended for initial treatment in pregnant or nursing rats or those less than 4 months old.

However, if symptoms are advancing, these stronger antimicrobials may be necessary. The benefits of using one of these drugs or a combination of drugs may outweigh the risks. Discuss with your veterinarian.

Additional Treatments for Rat Respiratory Infections

If your rat is experiencing respiratory distress (e.g. gasping or labored breathing), and/or their gums, ears, feet, or tail appear to be blue-tinge (cyanotic) or is turning pale, oxygen therapy should be initiated. Consult your vet.

Keeping the rat hydrated will prevent dehydration and help to loosen secretions, making them easier to remove from the lungs. Providing high caloric foods and encouraging the rat to eat will help maintain the strength needed to combat the illness. Rats that are ill frequently tend to lose their appetites and stop drinking water. You will need to monitor them closely.

Nursing Care

  • Use a nebulizer with a particle size of 0.5-5 micrometers when nebulizing medication for rats. Nebulizers with a larger particle size will not be as effective in delivering the medicines to the rat.
  • Treatments may take from 10 – 30 minutes depending on the volume given and how well the rat tolerates the therapy. Observe for signs of increased agitation or intolerance. If noted, stop treatment and contact your vet.

Nebulizer Set-Up Used to Treat Rat Respiratory Infections

  • Use a humidifier, or stay with the rat in a closed heated mist bathroom for 10-15 minute intervals. This will soothe breathing passages and loosen secretions.
  • If your rat is heavily congested, cool mist humidifiers or vaporizers may be more comfortable.
  • Remember to clean humidifiers or vaporizers following each use to prevent the growth of organisms from standing water.
  • You can provide additional warmth to help maintain body temperature within normal limits. It is vital that the rat does not become overheated or dehydrated. The rat should also be able to easily move away from the heat source if it becomes too uncomfortable. If the rat is unconscious or immobile, keep the temperature low and stable. This type of care is best provided by a vet.
  • Utilize re-usable and re-heatable isothermic products. Make sure to follow the directions carefully and wrap it in a towel before placing it in the cage. And ensure the rats cannot chew on it. Some products provide heat for 12 hours before they need to be reheated. Other similar types of products may vary in reheat time. Check directions for the individual product.
  • If using an electric heating pad (long-term use), use only the lowest heat setting. Put a towel in between the heat pad and the cage bottom. Place it beneath an outside corner of the cage.
  • If these options are not available, use a plastic bottle filled with hot water wrapped in a towel. Place in the corner of the cage and replace as needed.


  • When administering antibiotics to your pet, include yogurt (with live active cultures) in their diet. This will prevent the destruction of normal gut bacteria.
  • Provide additional nutritional supplements such as soy baby formula, Ensure, Boost, NutriCal paste, and baby foods. If your rat is not willing to eat on its own, use a needleless oral syringe every 2 hours. Be careful to prevent aspiration. Giving small amounts of food in this fashion will help to promote intestinal motility during illness. Include a multi-vitamin supplement (pet store) if food intake is inadequate.
  • Place food and water close, and on the same level with the rat, to prevent it from exerting itself. Overexerting a rat who is already having difficulty breathing will stop the rat from wanting to eat and drink.
  • Provide plenty of fluids to help prevent dehydration. If your rat is willing to drink on its own or by a needleless syringe, we suggest freshwater or a glucose mixture. To make the glucose mixture, combine 3 teaspoons of honey per 1 pint of warm water. Ensure that the water is warm enough to dissolve honey and cool enough not to burn the rat’s mouth. You can also try electrolyte replacement drinks such as Pedialyte or Gatorade.
  • Please note that Pedialyte is only suitable for use for 24 hours after opening and must be refrigerated. It can be frozen and thawed when needed.
  • Ensure to prevent aspiration while giving fluids with an oral syringe. If the rat is not willingly drinking, discuss with a veterinarian.
  • Unless there is undue stress, allow your rat to remain with cage/littermate for comfort.
  • Contact your veterinarian to discuss changes in treatment in the event the condition does not seem to be improving. If the situation continues to deteriorate, and precludes further comfort or quality, discuss euthanasia with a veterinarian.


  • Airway remains clear.
  • Nutrition level adequate.
  • Maintains adequate weight for the size of the rat.
  • Comfort and quality of life maintained.

Prevention of Rat Respiratory Infections

To date, the elimination of Mycoplasma is virtually impossible. Research Labs, through cesarean section and strict bacterial barrier maintenance, have been able to reduce infection and produce Mycoplasma free rats for breeding.

But for the typical pet rat owner, the focus should be on suppressing clinical signs and symptoms. You can achieve this some degree by:

  • Breed from rats with a higher Mycoplasma resistance.
  • Early treatment recognition and treatment of symptoms as appropriate.
  • Prevent severe symptoms by changing bedding and cleaning the cage frequently.
  • The use of dust and phenol-free bedding and litter will help reduce clinical symptoms.
  • Locate cage or housing in a draft-free area.
  • Refrain from smoking, vaping, or burning candles.
  • Avoid the use of fabric softeners or scented soaps.
  • Quarantine new rats for a minimum of two weeks before introducing them into an existing colony.

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